Plural electric motor control systems

ABSTRACT

An electric motor control system, in which: two coacting electric motors, each having a rotatable armature and rotatable casing and field assemblies, are arranged to afford infinitelyvariable speed-control of members driven by them jointly; and the motors are interconnected mechanically and electrically in such manner that, by operation of electrical control means in circuit with the motors, their respective armatures and field assemblies are enabled to rotate at different speeds and, during part of the acceleration and deceleration phases of operation, in opposite directions relative to each other.

United States Patent Hender NOV. 6, 1973 PLURAL ELECTRIC MOTOR CONTROLSYSTEMS Inventor: Berkeley Stephens Hender,

Abergavenny, England Filed: June 7, 1972 Appl. No.: 260,431

US. Cl 318/8, 318/45, 318/48 Field of Search 318/8, 45, 48

References Cited UNITED STATES PATENTS 12/1964 Roe 10/1972 Middlebrookr.3l8/8X Primary ExaminerT. E. Lynch Att0rneyJoseph F. Brisebois et al.

[5 7 ABSTRACT An electric motor control system, in which: two coactingelectric motors, each having a rotatable armature and rotatable casingand field assemblies, are arranged to afford infinitely-variablespeed-control of members driven by them jointly; and the motors areinterconnected mechanically and electrically in such manner that, byoperation of electrical control means in circuit with the motors, theirrespective armatures and field assemblies are enabled to rotate atdifferent speeds and, during part of the acceleration and decelerationphases of operation, in opposite directions relative to each other.

7 Claims, 5 Drawing Figures PATENTEDNUV 6 I975 SHEET 10F 2 IllllIIILLLHIII] [I PATENTEDNM 6m 377L032 .SHEET 2 [1F 2 PLURAL ELECTRICMOTOR CONTROL SYSTEMS This invention relates to electric motor controlsystems, and (although having various other potential applications) isparticularly applicable to electricallydriven road vehicles andindustrial trucks, electric locomotives and other railway equipment, andelectric cranes and lifts.

The aim of the invention is to provide a low-loss electric motor controlsystem of simple character, which is continuously variable and which iscapable of being employed over very wide ranges of voltage and current,such as, for example, might arise with homopolar motors.

Basically, the invention is an electric motor control system in which:two coacting electric motors, each having a rotatable armature androtatable casing and field assemblies, are arranged to affordinfinitelyvariable speed-control of members driven by them jointly; andthe motors are interconnected mechanically and electrically in suchmanner that, by operation of electrical control means in circuit withthe motors, their respective armatures and field assemblies are enabledto rotate at different speeds and, during part of the acceleration anddeceleration phases of operation, in opposite directions relativeto eachother.

Otherwise expressed, the invention provides a means of controlling therespective drives from two electric motors to two output shafts whichdrive wheels or other machinery; the outputs from the shafts beingmechanically and electrically correlated to enable the shafts to operateat different speeds relative to each other, and to provide the fullrange of speed-control from standstill to maximum speed by means of alow-loss control system.

Referring to the accompanying drawings:

FIG. 1 illustrates schematically one embodiment of the invention;

FIG. 2 illustrates schematically another embodiment of the invention;

FIG. 3 depicts schematically a modification of the embodiments shown inFIGS. 1 and 2;

FIG. 4 shows a basic electrical circuit which is applicable to any ofthe schemes illustrated in FIGS. 1 to 3; and

FIG. 5 represents another electrical circuit which can be used with anyof the schemes illustrated in FIGS. 1 to 3, as an alternative to thecircuit shown in FIG. 4.

In the embodiment illustrated in FIG. 1, an electrically driven vehicle(e.g. a battery electric road vehicle or an industrial truck) has twodriven wheels W1 and W2 connected, by drive-shafts S1 and S2respectively, to casings Cl and C2 of two coacting direct currentelectric motors M1 and M2. Each of the motor casings C l and C2 is soarranged as to be capable of rotating, and to receive power throughbrushes B (only one set of which is shown). Each of the motors M1 and M2has its armature A connected (by an output shaft S3 or S4), to areversing gearbox RG through gears l and 2; and, to provide a suitablerotational speed for the wheels W1 and W2, the motors are preferablyfitted with epicyclic gearing EG (or any other form of speedreductiongearing). The reversing gearbox RG has its housing fixed to a member Fof the frame of the vehicle.

By operation of electrical control means (described later), therespective armatures A and field assemblies of the motors M1 and M2 areenabled to rotate at different speeds and, during part of theacceleration and deceleration phases of operation, in oppositedirections relative to each other.

When the two motors M1 and M2 are equally energized, the output shaftsS3 and S4 to the gearbox RG have torques which oppose each other throughthe gearing, and therefore the casings Cl and C2 of the motors areforced to rotate so as to drive the wheels W1 and W2 in the samedirection, and at the same speed if the vehicle is travelling in astraight line.

The embodiment depicted in FIG. 2 differs from that of FIG. 1 by havingthe motors M1 and M2 physically reversed; so that the drive-shafts S1and S2 of the wheels W1 and W2 are driven by the corresponding epicyclicgearing EG, and the casings C1 and C2 of the motors are connected to,the reversing gearbox RG. But the operation of the motor control systemof FIG. 2 is similar to that of FIG. 1.

FIG. 3 shows a modification which enables the reversing gearbox RG to beeliminated. In this arrangement the casings C1 and C2 of the motors M1and M2 are fitted with gears G which directly couple one with the other.Otherwise the system operates in the same manner as that of FIG. 2, thedrive-shafts S1 and S2 being connected to the respective wheels W1 andW2 (or other rotatable members).

The basic electrical circuit (FIG. 4), which is applicable to any of theschemes shown in FIGS. 1, 2 or 3, has two series-wound motors M1 and M2,with their field windings F1 and F2 respectively, connected in serieswith each other across a DC'supply. A switch SW1 controls the supply toboth motors, but a switch SW2 shortcircuits the supply to the motor M2.The strength of the field of field winding F2 is controlled by avariable shunt resistance R. If the system is started with the switchSW2 closed, no field can exist in the motor M2 and therefore no torquecan be derived by it. Consequently, the motor Ml alone operates androtates the armature of the motor M2 in the opposite direction to thatin which it would normally rotate when the motor M2 is fully energized;this armature, in effect, merely acting as a flywheel. If the resistanceR is reduced to its minimal value and the switch SW2 is opened, thecurrent flowing through the motor M1 will then flow through the armatureof the motor M2, and partly through the field winding F2 and partlythrough the resistance R, so that a small torque will appear in themotor M2. The same torque will also appear in the motor M1 which must beequal in torque-rating to the motor M2 at all times.

.If the resistance R is gradually increased, the current in the fieldwinding F2 increases steadily and the torque in the motor M2 builds upbut its armature is continuing to rotate in the opposite directionrelative to the field assembly to that in which it will ultimatelyrotate relative to the field assembly; and, in consequence its armaturewill act as a generator, adding voltage to the DC supply to drive themotor M1. At a certain point of the regulation of the resistance R, thestrength of the field F2 of the motor M2 will be sufficiently great toproduce a torque which will stop the armature of this motor fromrotating relative to its own field assembly; and still further increasein the strength of the field F2 will cause the armature to rotate in thenormal fully energized direction relativeto the field assembly. When thefield winding F2 is fully connected to the supply with the resistance Rfully out of circuit, the torque will build up to its maximum and bothmotors will run under equal conditions and in the correct direction fortravel of the vehicle wheels W1 and W2 (or for rotation of othermembers). Thus this system provides a continuously variable control forthe motors M1 and M2 from the standstill condition to maximum speed. Thekinetic energy of the motors when started, and with the vehicle atstandstill or near standstill conditions, assists the syste to drive thevehicle without imposing heavy peak power demands on the motors, thusrelieving the power supply system from peak current demands.

In practice, it may be found that the circuit arrangement shown in FIG.4 entails some difficulty in the design of the resistance R owing to thenormal inherently low resistance of the field winding F2. Suchdifficulty, however, can be obviated by adopting the circuit arrangementshown in FIG. 5. Here the motor Ml has a field winding SHl, and themotor M2 has a field winding SH2. The field winding SHl may beshuntconnected or it may be compounded or remain seriesconnected (as thefield F1 in FIG. 4), but the field SH2 of the motor M2 is under theinfluence of a DC chopping circuit which may be transistor-controlled orthyristor-controlled by already well-established means. In this instancethe chopping circuit, including for example a rectifier D shunting thefield SH2, has two transistors TI and T2 with associated resistors R1and R2 connected as shown. The strength of the field SH2 can becontrolled by means of the chopping circuit and may be under theinfluence of a sensing device in the motor circuit, so as to relate thestrength of the field SH2 to the demand of the motor M2. If desired, thefield SI-ll of the motor Ml can be simultaneously influenced by thechopping circuit so as to provide the required overall torque speed andpower characteristic to drive the load. 7

In this circuit arrangement the switch SW2 shorts out the armature ofthe motor M2 for starting purposes,

and, at the appropriate time, this switch is opened to 7 allow thearmature of the motor M2 to build up torque under the influence of thefield provided by SH2. The circuits are under the influence of theswitches SW1 and SW3, which are employed to isolate the motor and thefield circuits from the DC supply when necessary.

It will be appreciated that several variations of the circuitarrangements are possible, and those described are examples toillustrate the working of the invention. It is also desired to point outthat when the output shafts of the system are driving (as in the case ofa vehicle travelling downhill or being decelerated), the motors may bemade to act as generators and to return energy to the battery or othersource of energy.

It is possible for the system to be regenerative over a wide speed rangethrough the ability of the vehicle operator to increase the fieldstrength in each of the motors M1 and M2 simultaneously orindependently, thus causing the motors to increase their back e.m.f. toexceed the voltage of the supply and to feed current back into thatsupply, thus providing electrical braking. This could occur down toapproximately one-third of full speed. In applications requiring asingle drive-shaft output the shafts S1 and S2 may be mechanicallyjoined by a layshaft and gears, or by similar means.

The main advantages afforded by the invention may be summarised asfollows:

l. A continuously variable speed-control from zero to maximum speed,which may be applied to controlling the speed of a vehicle or othermachinery.

2. A continuously variable control system having an 7 inherently simpleinput control.

3. A control system which may be regenerative over a wide speed rangeand which, therefore, is capable of providing electrical braking.

4. A system of control suitable for a wide range of applied voltages,and therefore currents, since the main control device affects only thefield,and does not connect into the heavy-current or high voltagecircuitry.

I claim:

1. An electric motor system, comprising; a first electric motor and afirst driven member, said motor comprising first and secondindependently rotatable rotors, one of which constitutes a motorarmature while the other constitutes a casing and field winding, thefirst of said independently rotatable rotors being connected to drivesaid first driven member; a second electric motor and a second drivenmember, said second motor comprising third and fourth independentlyrotatable rotors, one of which constitutes a second motor armature whilethe other constitutes a second casing and field winding, said thirdindependently rotatable rotor being connected to drive said seconddriven member; intercoupling drive means between said second and fourthindependently rotatable rotors through which rotation of one of saidsecond and fourth rotors produces opposite rotation of the other,whereby, the reaction torque on said first motor due to the drivingthereby of said first driven member is sustained by the second motor,and vice versa; electrical circuitry connecting the armatures and fieldwindings of said first and second motors to a common electrical supply,and including switch means for removing the current supply from thearmature of said second motor without interrupting the supply to thefirst motor, and electrical control means for changing the field currentof said second motor.

2. An electtic motor control system according to claim 1, in which eachof the motors has its armature connected through gearing to a reversinggearbox, and has its casing arranged to drive the corresponding drivenmember.

3. An electric motor control system according to claim 1, in which eachof the motors has its rotatable casing connected to a reversing gearbox,and has its armature connected through gearing to the correspondingdriven member.

4. An electric motor control system according to claim 1, in which themotors have their casings coupled together by gearing, and each motorarmature drives the corresponding driven member.

5. An electric motor control system according to claim 1, in which thedrive to each of the driven members is transmitted by epicyclic gearingor other speedreduction gearing.

6. An electric motor control system according to claim 1, in which thetwo motors are series-wound and have their field windings connected inseries with each other across a DC supply; a switch is arranged so thatit can short-circuit the supply to one of the motors; and the same motorhas the strength of its field controlled by a variable shunt resistance.

7. An electric motor control system according to claim 1, in which thetwo motors are connected across a DC supply, and the field of one or ofeach of them is arranged to be influenced by a DC chopping circuit whichis electronically controlled.

1. An electric motor system, comprising; a first electric motor and afirst driven member, said motor comprising first and secondindependently rotatable rotors, one of which constitutes a motorarmature while the other constitutes a casing and field winding, thefirst of said independently rotatable rotors being connected to drivesaid first driven member; a second electric motor and a second drivenmember, said second motor comprising third and fourth independentlyrotatable rotors, one of which constitutes a second motor armature whilethe other constitutes a second casing and field winding, said thirdindependently rotatable rotor being connected to drive said seconddriven member; intercoupling drive means between said second and fourthindependently rotatable rotors through which rotation of one of saidsecond and fourth rotors produces opposite rotation of the other,whereby, the reaction torque on said first motor due to the drivingthereby of said first driven member is sustained by the second motor,and vice versa; electrical circuitry connecting the armatures and fieldwindings of said first and second motors to a common electrical supply,and including switch means for removing the current supply from thearmature of said second motor without interrupting the supply to thefirst motor, and electrical control means for changing the field currentof said second motor.
 2. An electtic motor control system according toclaim 1, in which each of the motors has its armature connected throughgearing to a reversing gearbox, and has its casing arranged to drive thecorresponding driven member.
 3. An electric motor control systemaccording to claim 1, in which each of the motors has its rotatablecasing connected to a reversing gearbox, and has its armature connectedthrough gearing to the corresponding driven member.
 4. An electric motorcontrol system according to claim 1, in which the motors have theircasings coupled together by gearing, and each motor armature drives thecorresponding driven member.
 5. An electric motor control systemaccording to claim 1, in which the drive to each of the driven membersis transmitted by epicyclic gearing or other speed-reduction gearing. 6.An electric motor control system according to claim 1, in which the twomotors are series-wound and have their field windings connected inseries with each other across a DC supply; a switch is arranged so thatit can short-circuit the supply to one of the motors; and the same motorhas the strength of its field controlled by a variable shunt resistance.7. An electric motor control system according to claim 1, in which thetwo motors are connected across a DC supply, and the field of one or ofeach of them is arranged to be influenced by a DC chopping circuit whichis electronically controlled.